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If I have a list:

A = [1,1,1,0,0,1,0]

How can I return the index of any occurence of the number 1?

With this example, how to return the following:

[0, 1, 2, 5]

This is because the number 1 appears specifically in these parts of the list

1 Answers1

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You can use a simple list comprehension:

A = [1,1,1,0,0,1,0]

i = [i for i,v in enumerate(A) if v==1]

[0, 1, 2, 5]
Pedro Maia
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  • Could you provide an explanation for how enumerate works here? Is it sort of like a second index? –  Dec 17 '21 at 13:00
  • It creates a list of `tuples` with each element and its associated index e.g. `enumerate([1,2,2,3])` would become `[(0,1),(1,2),(2,2),(3,3)]` the first number is the index and the second is the value. You can read more about it [here](https://docs.python.org/3/library/functions.html#enumerate) – Pedro Maia Dec 17 '21 at 13:04
  • How would this look in a function without enumerate? –  Dec 17 '21 at 13:34
  • You'd need to iterate over the range of the len: `i = [i for i in range(len(A)) if A[i]==1]` – Pedro Maia Dec 17 '21 at 13:38